This setting is currently unavailable for mobile prototypes. If you need to adjust the background color of a screen for a mobile prototype, you will need to update the background color within the design tool used to create the screen.
InVision pulls the bottom corner pixel colors and tries to match the background as closely as possible. In most cases, it's seamless; however, you can change the background color or use a background image for your desktop prototype screens as needed.
To change the background color or image for your screens in a desktop prototype:
Sign in to InVision and open the prototype and screen you want to update.
On the right side of the console toolbar, click the ••• (more) icon.
In the settings dialog, under the Background heading, update the color hex code or image. Optionally, click the Apply to all current and future screens checkbox (or do not click if you want to apply the change to only the screen that's open).